Preparation And Characterization Of Graded Cathodes For Solid Oxide Fuel Cells

Solid oxide fuel cells(SOFCs)are being considered as a great potential in power generation devices.Nowadays,one of the most important research goals is to develop intermediate temperature SOFCs to solve various problems associated with the high operating temperature.However,as the operating temperature is lowed from 900-1000 C to about 600 C,the conductivity of cell component materials decrease exponentially and interfacial polarization resistance at the cathode increase significantly which dramatically diminishes the output power density of fuel cell. Much effect has been devoted in improving the cathode performance by both using mixed ionic and electronic conductor materials and improving the micro-structural design at the cathode and the electrode/electrolyte(EE)interface.As a method of optimizing both the composition and structure of the cathode, functional graded electrode has been applied to SOFC in the recent years and offered great potential to enhance the SOFC performance.There are three kinds of grading:(1) composition grading,(2)particle size grading,(3)porosity grading.Composition and particle size grading is possible to increase electrochemical activity,optimize the electronic/ionic conductivity and tailor the mismatch between the physical properties of electrode and electrolyte materials.Porosity grading has potential to enhance the gas transport.Ideally,the best structure for a SOFC should be the one with compositional gradient and porosity gradient,uniform pore structure,consisting of fine grains close to the electrode/electrolyte interfacial,and large grains at air side.This thesis focuses on the above mentioned issues to carry out studies on designing and preparing graded cathodes.In this work,Sm_0.5Sr_0.5CoO₃(SSC)and Sm_0.2Ce_0.8O_1.9(SDC)composite cathodes with composition,porosity and particle size grading were fabricated utilizing spin-coating technique.Single cells were prepared using SDC as electrolyte and Ni-SDC as anode and tested in temperature range of 600-800 C.The graded cathode performance has obviously been improved comparing to that without grading structure.Current-Voltage curves shows that the output voltage of the single cell with graded cathode is 14.1%higher than that with non-graded cathode and the maximum power density also has a 13.3%increasing.